As much as I know, there is a limit for donating blood. You can not donate like 5L of your blood. Or you can not donate every day. Yes there is a limit for division. But note that blood cells continuesly die and the new ones are produced even if you don't donate.

"Red blood cells, white blood cells, and platelets grow from a single precursor cell, known as a hematopoietic stem cell. Remarkably, experiments have suggested that as few as 10 stem cells can, in four weeks, multiply into 30 trillion red blood cells, 30 billion white blood cells, and 1.2 trillion platelets—enough to replace every blood cell in the body." (from: http://encarta.msn.com/encyclopedia_761 ... d.html#p46 )
Looks good doesn't it?

Too much donation will have harmful effects, in my opinion. But a few won't hurt.